In optical transport networks a plurality of high speed data signals is transmitted as a (dense) wavelength division signal via optical fibers. Optical fiber amplifiers (EDFAs) or semiconductor amplifiers and in addition Raman amplifiers are used for signal amplification. Raman amplification is induced in the transmission fiber by one or more Raman pump signals injected into the transmission fiber usually against the signal propagating direction. The Raman amplification depends on the frequency difference between the Raman pump signal and the signal or channel frequency. The exact Raman gain obtained in the network is usually different from an predicted gain.
Several methods for measuring Raman gain are described so far:
US 2004/0120664 discloses a method and an arrangement for measuring Raman gain based on means of optical time domain reflectometry which is not suited for measurements with live traffic.
EP 1 508 985 proposes a solution based on a low frequency intensity modulation of a Raman pump signal. A corresponding intensity modulation of the optical signal is then measured. The Raman on-off gain is then determined from the intensity modulation in the optical signal.
EP 1 248 334 describes a method and a system for automatic in service gain control based on a spectrum analyser. The method and system permit in service measurements, permits derivation of the Raman gain and dynamic adjustments of system parameters, especially a power profile of received signals.